I used 1/2" plywood under 1/2" homasote with 3/8" Flexxbed with Atlas O on top for approximately one-third the layout. Then I experimented and used Atlas O on homasote on plywood for another third. Then finally Atlas O on plywood for the remainder. Know what? No option is not noticeably louder or quieter than either of the other options. But there was a noticeable difference to my wallet.
One feature (nice or not, depending on how you look at it) of Flexxbed that never gets mentioned - it is pliant, so if your sub-bed is slightly uneven in spots and you want to smooth out, or you want to create a slight super-elevation without much work, you can just selectively screw down certain screws harder and you can accomplish. Other side of that coin, if you want a perfectly level track (side to side or down the line), you can fail using Flexxbed if not vigilant of the amount each screw is tightened down.
Why did I go with 3/8"? Because Atlas O, like every other 3-rail track, is too tall. Having this track sit atop 1/2" roadbed and then ballasting that height makes it look cartoony to me. So for less money (thinner Flexxbed cost less and lower height means less ballast), I get a slightly more realistic look. Hmmmm.....
Peter